A typical electro-acoustic transducer such as a microphone or speaker includes a diaphragm. A typical microphone receives vibrations of a diaphragm in response to sound waves as electromagnetic changes, changes in capacitance, or optical changes, which are then converted into electrical signals. A typical speaker electromagnetically converts audio signals into vibrations of a diaphragm to output sound waves.
The diaphragms in these electro-acoustic transducers are used for mutual conversion between aerial vibrations and electrical signals. In other words, the electro-acoustic transducer has a diaphragm that connects audio, mechanical vibration, and electrical circuit systems. Such a diaphragm for the mutual conversion in any type of conventional and typical electro-acoustic transducers may cause poor frequency responses.
Thus, the inertial force due to the mass of the diaphragm, even of an extremely minimized one, in the conventional electro-acoustic transducer may result in a poor frequency-based sound collection.
An example electro-acoustic transducer having no diaphragm and detecting a particle speed by utilizing electric discharge to perform electro-acoustic transduction is disclosed in Japanese Patent Application Laid-Open No. 55-140400 (Patent Literature 1). The transducer described in Patent Literature 1 includes a needle discharge electrode and an opposite electrode surrounding the discharge electrode, the opposite electrode being apart from the discharge electrode. The opposite electrode is composed of a spherical conductive material having punched pores to propagate sound waves therethrough. The discharge electrode extends towards the center of the spherical opposite electrode. A high-frequency voltage signal is applied from a high-frequency voltage generating circuit to the discharge electrode, the high-frequency voltage signal being modulated with a low-frequency signal which is to be converted into sound waves. The high-frequency voltage signal causes a corona discharge between the discharge electrode and the opposite electrode, which emits the low-frequency signal, i.e., a sound wave.
The transducer described in Patent Literature 1 is an ion speaker, which converts an electrical audio signal into a sound wave using electrical discharge. The ion speaker described in Patent Literature 1 cannot be used as a microphone, and such a use is not suggested.
The present inventor has invented a microphone that includes a needle electrode; an opposite electrode facing the needle electrode; a discharge region defined between the needle electrode and the opposite electrode; a high-frequency oscillating circuit that includes the discharge region, the high-frequency oscillating circuit causing high-frequency discharge in the discharge region; a sound wave introducer that introduces a sound wave to the discharge region; and a modulated signal extractor that extracts a signal modulated in accordance with the sound wave which is generated in the high-frequency oscillating circuit and introduced to the discharge region.
The electro-acoustic transducer or microphone, which involves high-frequency discharge, generates a non-uniform electric field between the needle electrode and the plate electrode and applies a high-frequency high voltage to the non-uniform electric field to form plasma. The plasma is generated in the vicinity of the needle electrode in a high electric field and extends towards the plate electrode. The needle electrode in contact with the plasma is in a high temperature, which wears the tip of the needle electrode of the electro-acoustic transducer and leaves corona products at or near the tip. This results in deterioration of discharging characteristics, for example, a plasma length or shape. The deteriorated discharging characteristics cause an abnormal discharge, which results in a defect, such as an abnormal sound wave from the discharging region.
The present inventor has presumed that the substance in the air causes corona products at or near the tip of the needle electrode, and filed a patent application for an electro-acoustic transducer involving high-frequency discharge, the electro-acoustic transducer including a channel that supplies inert gas toward the perimeter of the needle electrode to prevent wear of the tip of the needle electrode and adhesion of corona products, and abnormal discharge during a continuous high-frequency discharge (See Japanese Patent Application Laid-Open No. 2011-188037).
An example configuration of the electro-acoustic transducer described in Patent Literature 2 is shown in FIG. 2. FIG. 2 illustrates a microphone unit 20 including a needle electrode 23 and an opposite electrode 24 to cause discharge therebetween. The needle electrode 23 has a cylindrical base which is covered by an insulating cylinder 26. The insulating cylinder 26 is further fit to an insulating cylinder 25. The insulating cylinder 25 is fit to a base 21 while penetrating therethrough in the width direction thereof. In other words, the base of the needle electrode 23 is fixed to the base 21 through the insulating cylinders 25, 26, while penetrating through the base 21 in the width direction thereof.
The base 21 corresponds to the bottom of a cylindrical case 22. The case 22 includes a flange at the perimeter of the open end. The tip of the needle electrode 23 is formed into a cone shape with a sharp end. The tip of the needle electrode 23 extends substantially coaxially with the case 22 and is located within the space defined by the case 22. The needle electrode 23 may be composed of tungsten having a tip curvature of 50 μm.
An opposite electrode 24 covers the open end of the case 22, the open end being opposite to the base 21, which corresponds to the bottom of the case 22. The opposite electrode 24 is a plain electrode. The opposite electrode 24 is composed of, for example, a punched metal sheet having numerous pores or a conductive wire net so as to propagate sound waves therethrough. The opposite electrode 24 is covered with an insulating material. The opposite electrode 24 may be composed of, for example, a stainless steel sheet having a lot of pores to propagate sound waves therethrough, the stainless steel sheet being covered with ceramic (silica) having a thickness of 0.1 mm.
The opposite electrode 24 faces the tip of the needle electrode 23 at a certain distance therebetween, and the opposite electrode 24 and the needle electrode 23 define a discharging region. The discharging region, in which high-frequency discharge is generated, is included in a high-frequency oscillating circuit. The discharge or the torch discharge generates a torch flame 27. The opposite electrode 24 defines a sound wave introducing region that introduces sound waves into the discharging region in the case 22 as described above. The peripheral wall of the case 22 may have pores to introduce sound waves to the discharging region in the case 22.
To cause a high-frequency discharge between the needle electrode 23 and the opposite electrode 24, a high-frequency high voltage needs to be applied thereto. The high-frequency oscillating circuit thus has a vacuum tube as an active oscillating element, the vacuum tube being capable of withstanding a high voltage. The high-frequency oscillating circuit is configured such that the discharged current running through an electrical discharge channel between the needle electrode 23 and the opposite electrode 24 returns to the circuit. In other words, a high-frequency self-oscillating circuit is formed.
The needle electrode 23 and the opposite electrode 24 define a high-frequency discharge region. The particle speed in the high-frequency discharge region depends on the particle speed of a sound wave, and thereby equivalent impedance changes. The equivalent impedance in the high-frequency discharge region depending on a sound wave allows a signal from the high-frequency oscillating circuit to be modulated by the sound wave. The modulated signal includes frequency modulated (FM) components and amplitude modulated (AM) components, the number of the FM components being larger than that of the AM components. The FM signal is extracted and input to a frequency demodulating circuit to be converted into an audio signal in response to the sound wave from the sound wave introducing region.
The needle electrode 23 is surrounded by an inert gas guide channel 251 up to the vicinity of the tip thereof. The inert gas guide channel 251 is provided to carry the inert gas along the peripheral surface of the needle electrode in the electro-acoustic transducer. The inert gas guide 251 is composed of a cylindrical insulating material, the 251 being integrated with the insulating cylinder 25 that holds the peripheral surface of the insulating cylinder 26, the insulating cylinder 26 supporting the needle electrode 23. The inert gas guide 251 surrounds the needle electrode 23 with a space therebetween, and includes a gas flow outlet 252 through which the inert gas flows along the peripheral surface of the tip of the needle electrode 23. The tip of the needle electrode 23 extends through the gas flow outlet 252, which achieves the discharge between the needle electrode 23 and the opposite electrode 24.
The inert gas guide channel 251 is in communication with a pipe 42. The pipe 42 is fixed to the cylindrical case 22 while extending therethrough in the radial direction. A first end of the pipe 42 is fit to the inert gas guide 251. This configuration allows the internal space of the pipe 42 to be in communication with the inert gas guide channel 251.
A second end of the pipe 42 resides outside the case 22 and is connected through a coupling 41 to a pipe 40. In addition to the above-mentioned inert gas guide channel 251, the pipe 40, coupling 41, and pipe 42 define an inert gas supply channel from a gas cylinder (not shown) to the needle electrode 23 of the microphone unit 20 or the electro-acoustic transducer. Example inert gases used include helium or nitrogen gas.
The technique in Patent Literature 2 causes a high-frequency discharge between the needle electrode 23 and the opposite electrode 24 to achieve electro-acoustic transduction, while inert gas is supplied from the inert gas supply channel. The inert gas introduced through a supply channel including the pipe 40, the coupling 41, and the pipe 42 into the inert gas guide channel 251 flows through the inert gas guide channel 251 to the gas flow outlet 252. The inert gas focused through the gas flow outlet 252 flows along the perimeter of the tip of the needle electrode 23, which prevents the perimeter of the needle electrode 23 from contacting with air.
The technique in Patent Literature 2 can prevent corona products resulting from substance in air from adhering to the needle electrode 23. The technique in Patent Literature 2 can prevent an abnormal discharge caused by the corona product adhered to the needle electrode 23, thereby obtaining an electro-acoustic transducer involving a stable high-frequency discharge. The technique in Patent Literature 2 can also prevent wear of the tip of the needle electrode due to an air flow, which has not been achieved by the conventional technique.